1. Field of the Invention
The present invention relates to a method for low pressure thermoforming uniformly thin carriers at optical quality.
2. Description of the Related Art
A semi-finished lens that has been ground and polished to a specific prescription loses the standard protective coating on its back surface. A technique known as Back Side Transfer (BST) of Hard Multicoated Lenses (HMC) is one available process to add a coating onto the ground and polished back surface of such semi-finished lenses. BST is detailed in U.S. Pat. No. 6,562,466 which is assigned to the present applicant. In summary, a protective coating is applied to a thin plano carrier and brought into contact with the ground and polished back surface that has been covered with an optical adhesive. The carrier is compressed against the adhesively coated back surface by an air bladder and held in place until the adhesive cures. The carrier is subsequently removed, leaving the protective coating adhered to the back surface. The quality of such protective coatings is heavily dependent on several characteristics of the carrier. More particularly, the key characteristics of the carrier are: its thinness which contributes to its conformity to the back surface; its optical quality even though it is removed from the final lens; and the uniformity of thickness across the carrier's surface.
Previously, relatively simple carrier thermoforming methods, such as drape forming, matched mold, vacuum forming, and free blown forming have produced carriers of only mediocre quality. A high pressure thermoforming method described in U.S. Pat. Nos. 5,108,530 and 5,217,563 produces carriers of acceptable quality for a limited range of sizes and geometries. However, the high pressure method requires complex systems to control the heat and pressure that develop within the large scale equipment. As the mass of the equipment rises, it becomes increasingly difficult to control the temperature and other process parameters which affect quality control. Ultimately, the difficulty and expense of operating such high pressure systems outweighs the benefits and yields of acceptable carriers. Accordingly, it would be desirable to provide a simpler method utilizing low cost equipment to provide a greater yield of carriers in a larger range of sizes and geometries.